A Multidisciplinary Study to Define a Composite Hydrogeological System in a Rift Basin (Example of the Albertine Rift, Uganda)
Yves GERAUD, Marc DIRAISON, Daniel BARTIER, Yann HAUTEVELLE, Bastien WALTER, François RAISSON (GeoRessources, Université Lorraine)
[Ecole Nationale Superieur De Geologie De Nancy, France]
Regional fault structures along rift basins play a crucial role in focusing fluid circulation in the upper crust. The Albertine rift is known to present several active geothermal surface manifestations and constitutes a nice case study. Understanding the fluid flow at the scale of the basin is needed to estimate it potential. To define the hydrogeological system, four elements must be defined: the different sources, the plumbing, the reservoirs and the pressure gradients. The major Toro-Bunyoro fault system, bounding to the East the Albertine Rift in Western Uganda, hosts local fluid outflow zones within the faulted basement rocks, one of which is the Kibiro geothermal prospect. This major fault system represents a reliable example to investigate the hydrogeological properties of such regional faults, including the local structural setting of the fluid outflow zones. This study investigated five sites, where current (i.e. geothermal springs, hydrocarbon seeps) and fossil (i.e. carbonate veins) fluid circulation is recognized. This work used a multidisciplinary approach (structural interpretation of remote sensing images, field work, geochemistry) to determine the role of the different macroscale structural features that may control each studied fluid outflow zones, as well as the nature and the source of the different fluids. The local macroscale structural setting of each of these sites systematically corresponds to the intersection between the main Toro-Bunyoro fault system and subsidiary oblique structures. Inputs from three types of fluid reservoirs are recognized within this fault-hosted hydrogeological system, with “external basin fluids” (i.e. meteoric waters), “internal basin fluids” (i.e. hydrocarbons and sediment formation waters) and deep-seated crustal fluids. This study therefore documents the complexity of a composite hydrogeological system hosted by a major rift-bounding fault system. The rift-bounding Toro-Bunyoro fault system represents a discontinuous barrier for fluids where intersections with subsidiary oblique structures control preferential outflow zones and channel fluid transfers from the rift shoulder to the basin, and vice versa. Finally, this work contributes to the recognition of structural intersections as prime targets for exploration of fault-controlled geothermal systems. Thus, the fluids involve in geothermal system have three main origins, but their relative contributions depend on the stress field applied on the different reservoirs.
|        Topic: Geology||Paper Number: 12185|